Changes in Hemolymph Parameters in the Manila Clam Ruditapes philippinarum (Mollusca, Bivalvia) Following Bacterial Challenge

Abstract

Manila clams, Ruditapes philippinarum, were experimentally challenged with a pathogenic bacterium, Vibrio P1, the causative agent of Brown Ring Disease. Changes in cellular and humoral hemolymph parameters were studied over a 7-week period. Hemocyte concentration increased to a maximum 2 weeks after exposure of the clams to the Vibrio and was considered as hemocytosis in response to the pathogenic stimulus. The hemocyte concentration decreased thereafter and low values were maintained up to 7 weeks. A significant rise of a peptidase activity was observed in the cellular fraction of hemolymph 3 days postchallenge. A peak of serum activity, 1 week postchallenge, probably reflected the release of this peptidase by hemocytes. However, the serum protein concentration in challenged clams did not show any significant variation. There was a conspicuous shift between the early enzymatic response in the cells and the rise of hemocyte concentration, which suggested that the cellular mobilization and the enzymatic responses are controlled by distinct mechanisms. Low peptidase activity levels in both hemocytes and serum were observed 2 weeks following challenge, whereas the number of clams exhibiting the symptoms of Brown Ring Disease increased, indicating the development of the disease. Indeed, both hemocyte concentrations and leucine-aminopeptidase-specific activity in infected clams decreased with increasing infection stages. The depletion of the hemolymph parameters in the late phase may be a consequence of physiological stress due to an harmful effect of Vibrio P1.